Apparatus and method for intensifying illumination brightness by time-superposing multiple pulsed light sources

ABSTRACT

An illumination apparatus including: at least two light sources, in which each of the light sources produce independent light beams; a controller for sequentially driving each of the light sources at a high power above their respective maximum rated power, to produce a respective light beam for each light source, and for leaving the remaining light sources at a low power below their respective maximum rated power, such that the time-average of the high and low power levels are set to a predetermined value for each of the light sources; and a combiner and director for sequentially combining each of the light beams from their respective light sources while being driven at high power into a common output beam with a fixed direction. Various combiner and directors are disclosed including tiltable mirrors under the control of the controller as well as optical systems.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed to optical illumination systems andmore specifically to optical illumination systems that incorporatetime-modulated light sources and recombining modulators to increasebrightness.

2. Description of the Related Art

It is well known in the art that the brightness of a light source cannotbe increased by a passive optical system. Here “brightness” is used inthe technical sense of optical power per unit emission etendue, whereemission etendue is the product of solid angle in the emitted directiontimes source area measured in a cross-section perpendicular to emitteddirection.

Just as a non-attenuating optical system must preserve sourcebrightness, so it must also preserve as invariant the product of thesolid angle of the illuminating light and the cross-section area of thefocused illumination beam. The divergence (or convergence) of the beamcan be decreased if minimum beam diameter is allowed to increase.Conversely, minimum beam diameter can be decreased if the beam is mademore divergent (or convergent). However, it is only possible to makeboth improvements simultaneously if part of the beam is blocked, whichreduces collected power. For given fixed source brightness, the lightreceived by an illuminated object of fixed area is thus determined bythe solid angle that the illuminating light occupies. A geometricallyequivalent statement is that, for given fixed source brightness, theoptical power projected by illuminating optics of fixed diameter isdetermined by the solid angle into which the optics project the light.The optical system designer must ensure that the source is powerfulenough to radiate with this fixed brightness into all regions within thelens diameter, and into all directions within the output solid angle,and that the diameter and solid angle are well chosen according to thevarious constraints of the application. However, once the sourcebrightness, diameter, and solid angle are fixed in this way, thedesigner can only increase the delivered power by minimizing absorptionand scatter within the system; he cannot redesign the system toconcentrate more power into the limited diameter and solid angle.

These constraints of fundamental physics significantly limit the opticaldesigner's freedom to increase illumination intensity. For example, thefollowing equation shows that even if an illumination system can collectall rays emitted by a source of width S, maximum possible deliveredpower is achieved once one chooses the source large enough that|S|≧|Pα|, where P is the lens diameter and α the angle that the opticalsystem projects into. When this condition is satisfied, the lensaperture is completely filled by the source, and maximum intensity isdelivered within the projection angle α. Unfortunately, if S isincreased beyond the point needed to fill the lens, the overfillinglight cannot be collected, and the extra light that is output by thelarger source is therefore wasted. On the other hand, when |S|<Pα|, thesource is too small to fill the aperture, and illumination intensity canbe increased by increasing the source size, which, for a fixed class oflight source, means increasing the power consumption of the source.Image intensity is said to be power-limited in this case.

However, once |S|>|Pα|, further increases in source power do notincrease image intensity because the additional source area is notcollected within the lens aperture. Image intensity in this case is saidto be brightness-limited. Loosely speaking, one might say that when thesource is brightness-limited, image intensity can only be increased byincreasing the brightness of the collected rays; increasing the size ofthe emitting region to produce ‘more rays’ does not help.

The field size or angle α is often fixed by the application. To increaseimage intensity once the brightness limit is reached, the designer canincrease the lens diameter P (or equivalently, increase the numericalaperture [NA], defined essentially as the ratio of lens aperture radiusP/2 to object distance). However, technical constraints on lensperformance and/or practical constraints on cost often limit thefeasibility of increasing the lens diameter. This is particularly truein projection optical systems, where the illuminated object is re-imagedby a projection lens. High quality projection lenses must not only bedesigned to capture the full angular and spatial extent of the lightthat is reflected or transmitted by the illuminated object, they mustalso project a high resolution image of the object using this light.Image aberrations increase as lens diameter is scaled up. Resolutionrequirements are particularly stringent in photolithography systems. Inprojection displays the optics frequently include elements for color andpolarization separation/recombination whose cost scales very unfavorablywith NA. Thus, in photolithography projectors or projection displays itis not easy to increase the NA of the projection system.

Of course, one requirement for maximizing brightness is that thebrightest available source be chosen for the system, which essentiallymeans using the source that produces the greatest intensity on eachcollected ray. It is common practice to use arc lamps in applicationsthat demand high intensity within a limited NA or object size. It iswell known that arc lamps are the brightest light sources available,with the important exception of laser sources. From the point of view ofgeometrical optics, a laser can be considered to be a true point source,i.e. a source having infinitesimal extent, so that optical systems usinglaser sources are always power-limited and never brightness-limited.Practical issues with laser sources are often cost and size,particularly as power levels rise into the 1-Watt regime and above.Compact arc lamps in the 1000-Watt range can cost several hundreddollars and might occupy ˜200 cubic inches in the illuminator (plusremote power supply). Depending on the lamp, the portion of the consumedpower radiated as visible light might be 200 Watts. The cost of a laserin the 200-Watt range might be tens or hundreds of times that of thelamp, and the laser might occupy tens or hundreds of times the volume.Though the situation may change in the future, for many applicationslaser sources are often severely underpowered when practical constraintsare enforced on cost and size. On the other hand, while practicalnon-laser sources can provide very high power, they do so from anextended emitting region, which means that in many applications thepower they actually deliver does not reach ideal levels before abrightness-limited regime is reached.

What is needed is a way to increase the brightness of the emittingregion itself. However, commercial high brightness light sources areusually engineered to generate as much energy within the emission volumeas is technologically possible. For example, when an arc lamp issteadily powered above its rated level, its lifetime decreasescatastrophically (i.e. dropping from hundreds or thousands of hours to afew hours). Steady output at increased power requires that the lamp musthave a larger arc gap; this means that the source is increased in sizebut not in brightness.

Brightness can often be increased for brief intervals, but theapplication must permit the increased emission to accomplish its purposebefore damage mechanisms in the source are initiated by the acceleratedoperation. The source must then be switched off for a sufficientinterval to hold time-averaged power below the maximum rated level. Itis known in the art that total visible light emission can be improved bypulsing a metal halide lamp, even though total power consumption is heldfixed in the time-average, but for simplicity we will assume thattime-averaged visible light output is neither increased nor decreased bypulsing. For example, in color-sequential displays it is known that onecan periodically pulse LED sources in a way that holds theirtime-averaged emitted power within tolerance, but which alternatesperiods of intense emission with periods of non-emission in which thedisplay can be reset for the next color or color bit. Similarly, inphotolithography it is known that if one keeps lamp emission very lowduring periods in which the shutter is closed (for example, whilesilicon wafers are being loaded, aligned, or stepped to the next chipexposure position), one can cycle the intensity to a higher than normallevel during the actual expose period (duration usually <1 second).However, this is not useful in applications where light is requiredcontinuously.

Another approach that can provide limited increases in brightness has todo with a simplification made in the above discussion of sourcebrightness in an optical system. The emitting region of a source doesnot usually have uniform brightness or sharply defined edges. Forexample, the emitting region of an arc lamp is roughly defined by thegap between discharge electrodes (perhaps ˜2 mm), but source brightnessis usually highest near the electrodes and falls off in the middle ofthe gap (as well as decreasing radially outward). The lamp reflectoroften increases this brightness non-uniformity. In a real system thereis usually not a sharp transition between the power-limited andbrightness-limited regimes, making it sometimes preferable for thedesigner to choose a source large enough that some of the dimmer lightin the outer regions of the source is not collected, in order toincrease the size of the central high brightness region. Thus, thedecision about how to choose a source which best matches into the opticsinvolves a tradeoff between efficiency and total collected light.However, as source size is increased this tradeoff becomes increasinglyunproductive until a purely brightness-limited regime is reached.

Other techniques for intensity increase have to do with combining twosources, or combining two images of a single source. As per the abovediscussion, there is a fundamental physical limitation that reduces thebenefit attainable from such combination techniques. It is impossible tomerge two incoherent rays that propagate from different points or indifferent directions into a single ray with twice the energy, unless theinitial pair have different wavelengths, or are in differentpolarization states. A limited exception arises if one of the rays isgenerated by a source that is not opaque. However, in practice thispossibility proves difficult to exploit; for example recombination of apolarization-converted beam with the unconverted component by re-imagingit through the arc is typically not found to be very efficient. Thus,two sources that are unpolarized or of matched polarization can only becombined into a single effective source if the combined source is madetwice as large, or is made to radiate into twice as large an angle [orsome combination thereof]. Such a doubling of beam width or directionaldivergence is not useful in a brightness-limited situation. If anunpolarized source is to be used in an application requiring polarizedlight, the designer can arrange to separate the unpolarized source beaminto separate beams of opposite polarization, and can then convert thepolarization of one of the beams to match that of the other, both beamsthereby emerging in the polarization needed for the application. Thiseffectively doubles the source power in the desired polarization.However, for fundamental reasons it is impossible to merge the two beamsinto a common beam of doubled power but unchanged width and divergence.As noted above, in practice there is not a sharp division between thepower-limited and brightness-limited regimes, and the designer can oftenarrange for rays from a high brightness region of the converted beam todisplace rays from a low brightness region of the other beam. Thisimprovement is not as large as the 2× increase that would be obtained ifthe two sets of rays could actually be merged, but average brightness isincreased somewhat.

FIGS. 1a and 1 b show a known arrangement for effecting this conversionand recombination. (In a working system the FIG. 1a optics wouldtypically be followed by additional illumination optics, a target, andoptics to project an image of the illuminated target.) Light source 100is of a well-known kind, consisting of an arc lamp 102 with curvedreflector 104 (such as a paraboloid) that projects the emitted light asa beam. Light source 100 can alternatively include a lens (not shown) tocollimate or focus the output beam. Alternatively, such collimating orfocussing functions can be carried out by the reflector 102 alone. Lightsource 100 projects an unpolarized beam 106 into a polarizingbeamsplitter 108, hereinafter referred to as a PBS. Within PBS 108 apolarizing coating 110 divides beam 106 into perpendicularly polarizedcomponents; beam 112 with polarization out of the plane of the diagram(S polarization) and beam 114 polarized within the plane of the diagram(P polarization). Mirror 116 folds beam 112 parallel to beam 114, andbirefringent element 118 (most commonly a half-wave retarding plate)converts beam 112 to P polarization (matching the polarization of beam114); thus beams 112 and 114 are combined into a wider beam of commonpolarization. Lens 120 collects much of the polarized light from beams112 and 114, but in an application that is not power-limited, lens 120will not be wide enough to collect all the light. Increasing thediameter of lens 120 would require either increasing the NA of thefocused double beam, or increasing the size of the illuminated area atfocus. Note that light source 100 and lens 120 are not aligned with PBS108, nor is mirror 116 of the same length as polarizing coating 110, forreasons which may be understood from the simpler layout in FIGS. 1c and1 d. FIG. 1c shows, in schematic form, a plot 150 of the intensity thatis present in beam 114 across the diameter of lens 120 if light source100 and lens 120 are aligned with PBS 108, and if mirror 116 of the FIG.1a arrangement is removed. The length of dashed lines 122 and 152 inFIGS. 1b and 1 d, respectively, represent the diameter of lens 120,which is not wide enough to encompass all of beam 114 . FIG. 1b shows inschematic form, a plot 124 of the intensity across lens 120 produced bybeams 112 and beam 114 in the FIG. 1a arrangement where light source 100and lens 120 are not aligned with PBS 108, and where mirror 116 isshorter than the polarizing coating 110. The heights of dashed lines 122and 152 represent schematically, the average intensity levels (I) of theoutput beams. In the FIG. 1a arrangement, lens 120 collects less lightfrom beam 114 than in the FIG. 1c arrangement, but the lost light ismore than made up for by collection of the high brightness portion ofbeam 112. The part of beam 114 that is collected in the FIG. 1carrangement but not collected in the FIG. 1a arrangement has relativelylow brightness. This improvement, however, is not as large as the 2×increase that would be obtained if beams 112 and 114 could actually bemerged into a single (polarized) beam of unchanged width. Someimprovement is made by the FIG. 1a arrangement because brightnessnon-uniformities cause the collection in the FIG. 1c arrangement to beonly partly brightness-limited; it is partly power-limited as well. Ifone tried to accomplish the improved collection of the FIG. 1aarrangement using a source beam that was larger and more powerful (andtherefore more completely brightness-limited), the efficiency gain wouldbe less.

The output beam in the FIG. 1a arrangement is polarized. In the case ofunpolarized light, the simple arrangement shown in FIGS. 2a and 2 bsimilarly effects a combination of two partly brightness-limited beams,this time from two light sources, 200 a and 200 b. Non-symmetricalignment of light sources 200 a and 200 b with mirror 202 creates abrightness distribution, shown in FIG. 2b, similar to that of the FIG.1a arrangement. In the FIG. 2a arrangement, light beam 204 from lightsource 200 a is directed to the lens 208 simultaneously with light beam206 from light source 200 b which is first folded by mirror 202 and thendirected to the lens 208. If light source 200 a is not fullybrightness-limited in the application of interest, it can be combinedwith light source 200 b in the manner shown, but only at the cost ofdecreased efficiency, and as light source size is increased suchattempts to achieve greater collected power become even less efficient.In fact, the FIG. 2a arrangement is not very practical since the limitedincrease in collected power that it would provide is much the same aswould be obtained by simply using a single light source of larger power.Moreover, a light source of larger power often means a light source ofincreased emission volume that will provide little or no increase in thepower collectable by near-brightness-limited optics. For the samereason, the FIG. 1a arrangement may not actually provide a great dealmore light than the simple FIG. 1c arrangement; its advantage is oftenthat it allows a similar amount of light to be obtained with a smallerlight source. When such a smaller light source is used in the FIG. 1carrangement it is significantly power-limited; with a larger lightsource that is near brightness-limited, the FIG. 1a arrangementtypically provides only a modest increase in delivered power.

FIGS. 2c and 2 d present an even starker illustration of thesebrightness constraints. If surface 212 of PBS 210 is a PBS coating, beam218 will be unpolarized, and cannot have any higher intensity, shown inFIG. 2d, than would be provided by light source 200 a alone (with no PBS210), or by light source 200 b alone if surface 212 were a mirror.

What is needed is a way to collect as much power from two (or more)brightness-limited or near brightness-limited beams as would be obtainedwere it possible to combine two such beams into a single beam ofunincreased width and angular divergence; more generally, what is neededis to project more light from a fixed source volume into a fixed rangeof directions than is permitted by the operating power limits ofavailable compact sources.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an apparatus andmethod for intensifying illumination brightness in an optical systemwithout increasing the Numerical Aperture or lens of the system.

It is yet a further object of the present invention to provide anapparatus and method for intensifying illumination brightness in anoptical system without increasing the numerical aperture or lensdiameter of the system.

It is yet a further object of the present invention to provide anapparatus and method for intensifying illumination brightness in anoptical system by powering light sources above their rated power levelswithout damaging them or decreasing their life.

It is yet a further object of the present invention to provide anapparatus and method for intensifying illumination brightness in anoptical system by pulsing light sources to a power above their ratedpower level for applications which require continuous light.

It is still yet a further object of the present invention to provide anapparatus and method for intensifying illumination brightness in anoptical system which projects more light from a fixed source volume intoa fixed range of directions than is permitted by the operating powerlimits of available compact sources.

Accordingly, an illumination system is provided having two or moretime-modulated light sources. The illumination system is for use in anoptical system, for example, a projection display. A controlling unitsequentially cycles each source to high power, also ensuring that whenone source is run at high power, the other sources are off or at minimumpower. The duration of the high power phase is kept shorter than thetime constant for accelerated damage mechanisms in the sources, and thetime-averaged power level for each source is held within acceptableoperating limits. A recombining modulator is synchronized with thesources in such a way that the modulator sequentially passes each of thesource beams in turn into a direction in which a common output beam isformed. In some embodiments the recombining modulator sequentiallyswitches the polarization of each source into a common outputpolarization state.

The recombiner modulator is preferably engineered to deflect orrepolarize each source beam in a way that does not increase apparentsource size or beam divergence. For this to be possible it is afundamental physical requirement that the recombiner block all othersources when a particular source beam is being deviated into the outputbeam (because it is impossible to simultaneously merge two source beamsinto a common output beam); however, little light is blocked by thisaction because the other light sources are either off or operating atlow power during the portion of the cycle when light from a particularlight source is being passed. It should be noted that it is notnecessary that a source actually be driven at constant instantaneouspower during the phase of the cycle in which it is cycled high. Asdiscussed further below, it may be advantageous with certain sourcessuch as metal halide light sources to modulate the applied power at highfrequency during the on-phase.

If the time-averaged power at which each individual source is drivenequals the maximum safe operating power level, then even though theoutput beam width and apparent source size do not increase beyond thatfrom a single source, the time-averaged power projected into the commonoutput beam will be larger by a factor that can approach the totalnumber of sources.

In a preferred embodiment, given by way of example only, and not tolimit the scope of the invention, the illumination system can use firstand second pulsed arc light sources that illuminate the recombiningmodulator through different faces of a polarizing beamsplitter, onelight source illuminating the modulator in P polarization and the otherin S. The recombining modulator can be a switchable FLC cell withhalfwave retardation along a 45° axis in the on-state. When the secondlight source is cycled high, the modulator is switched to halfwaveretardation, rotating the output beam to P polarization. When the firstlight source is cycled high, the modulator is switched off, againleaving the output beam in P polarization. In time-average, each lightsource is driven at rated power, but the output power is doubled withoutincreasing beam width or divergence.

More specifically, the illumination apparatus comprises: at least twolight sources, each of which produce independent light beams; acontroller for sequentially driving each of the light sources at a highpower above their respective maximum rated power, to produce arespective light beam for each light source, and for leaving theremaining light sources at a low power below their respective maximumrated power, such that the time-average of said high and low powerlevels are set to a predetermined value for each of the light sources;and combining and directing means for sequentially combining each of thelight beams from their respective light sources while being driven athigh power into a common output beam with a fixed direction.

In a first embodiment of the illumination apparatus of the presentinvention the combining and directing means comprises an array oftiltable mirrors that sequentially direct each of the light beams fromtheir respective light sources while being driven at high power into acommon output direction.

In a second embodiment of the illumination apparatus of the presentinvention, the at least two light sources comprise first and secondlight sources in which the produced light beams from each light sourcehave a first and a second polarization, and wherein said combining anddirecting means comprises an optical system. In a preferred version ofthe second embodiment, the optical system comprises: a polarizingbeamsplitter for transmitting light of the first polarization andreflecting light of the second polarization; a fold mirror collectingeither the transmitted or reflected light from the beamsplitter anddirecting the collected light in a direction parallel to the otherlight; a half-wave retarder rotating either the transmitted or reflectedlight from the beamsplitter; and a condenser lens collecting theparallel transmitted and reflected light from the beamsplitter.

In a third embodiment of the illumination apparatus of the presentinvention, similar to that of the preferred version of the secondembodiment the light sources are configured such that the light beamfrom each respective light source is made to substantially focus in thevicinity of the fold mirror and condenser lens.

In yet another preferred version of the second embodiment of theillumination apparatus of the present invention, the optical systemcomprises: first and second sets of light guides corresponding to thefirst and second light sources, each individual light guide having twosubstantially parallel internal coated surfaces; first and second pairsof lenslet arrays corresponding to the first and second light sourcesfor focusing the light from individual lensets into a correspondingindividual light guide; a polarizing beam splitter array for directlytransmitting the light of the first polarization from each of the firstand second sets of light guides, and transmitting the light of thesecond polarization from the first and second sets of light guides afterreflection from the two internal coated surfaces, and half-waveretarding strips for converting either the directly transmitted light ofthe first polarization or the twice-reflected light of the secondpolarization to match the other of said polarization components.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages of the apparatus andmethods of the present invention will become better understood withregard to the following description, appended claims, and accompanyingdrawings where:

FIG. 1a illustrates a schematic of a prior art optical system forincreasing source brightness from a single light source.

FIG. 1b illustrates a graph of the intensity level of light across thelens of the optical system of FIG. 1a.

FIG. 1c illustrates a schematic of a prior art optical system having asingle light source.

FIG. 1d illustrates a graph of the intensity level of light across thelens of the optical system of FIG. 1c.

FIG. 2a illustrates a schematic of a prior art optical system forincreasing source brightness using two light sources.

FIG. 2b illustrates a graph of the intensity level of light across thelens of the optical system of FIG. 2a.

FIG. 2c illustrates a schematic of another prior art optical system forincreasing source brightness using two light sources.

FIG. 2d illustrates a graph of the intensity level of light across thelens of the optical system of FIG. 2c.

FIG. 3a illustrates a schematic of a first embodiment of an opticalsystem of the present invention having two light sources in which thefirst light source is powered and the second light source issubstantially unpowered.

FIG. 3b illustrates a graph of the intensity level of light across thelens of the optical system of FIG. 3a.

FIG. 3c illustrates a schematic of the first embodiment of an opticalsystem of the present invention having two light sources in which thesecond light source is powered and the first light source issubstantially unpowered.

FIG. 3d illustrates a graph of the intensity level of light across thelens of the optical system of FIG. 3c.

FIG. 4a illustrates a schematic of a second embodiment of an opticalsystem of the present invention having two light sources in which thefirst light source is powered and the second light source issubstantially unpowered.

FIG. 4b illustrates a graph of the intensity level of light across thelens of the optical system of FIG. 4a.

FIG. 4c illustrates a schematic of the second embodiment of an opticalsystem of the present invention having two light sources in which thesecond light source is powered and the first light source issubstantially unpowered.

FIG. 4d illustrates a graph of the intensity level of light across thelens of the optical system of FIG. 4c.

FIG. 5 illustrates a schematic of a third embodiment of an opticalsystem of the present invention having two light sources.

FIG. 6 illustrates an optical system of the prior art.

FIGS. 7a-7 d illustrate components of the prior art optical system ofFIG. 6.

FIG. 8a illustrates a fourth embodiment of an optical system of thepresent invention having two light sources.

FIG. 8b illustrates a portion of the optical system of FIG. 8a showingthe optical path of the first light source.

FIG. 8c illustrates components of the embodiment of FIG. 8a.

FIG. 8d illustrates a portion of the optical system of FIG. 8a showingthe optical path of the second light source.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIGS. 3a-3 d, there is illustrated a first embodimentof an illumination apparatus of the present invention, generallyreferred to by reference numeral 300. Illumination apparatus 300 is anembodiment of the present invention appropriate to optical systems thatuse unpolarized light. Unpolarized beams from first and second lightsources 302, 304 respectively, are combined into a single output beam306 that is also in the unpolarized state. Light sources 302 and 304 aredriven with an average power that is equal, for example, to the maximumrated power of the light sources. For example, with metal halide lightsources used in a projection display, this maximum power might be 250Watts. However, power supplies 308 and 310 actually cycle the lightsources 302, 304 between operation at 2× instantaneous power, such as500 Watts, and a powered-off condition where the light sources 302, 304draw almost no power. The duration of each cycle is preferably shortcompared to the thermal time constants (for example ˜0.1 sec.) of theelectrode structure of the light sources in order to ensure that theelectrodes do not overheat during the high power phase and that they donot cool sufficiently during the power-off phase to precipitate out themercury and metal halide additives in the light source, thus ensuringthat redischarge takes place when the next high power cycle begins.Color sequential projectors incorporating one lightvalve and three lightsources [red, green, and blue] in which a balance between red, green,and blue is achieved by driving each light source at an adjusted highlevel only when its color is being displayed, leaving it off otherwiseare known in the art. In such a projector, the time average power to theAC light source is held constant, and the power in positive pulses keptequal to the power in negative pulses. If the illumination apparatus 300of the present invention is used in such a projection display it isconvenient to modulate light sources 302, 304 at the refresh rate of thedisplay, which is commonly 60 Hz. A further step for ensuringredischarge is to keep the light sources 302, 304 running at low powerbetween high power cycles instead of turning them completely off, so asto maintain ionization in the gas. Both light sources 302, 304 can bepreferably warmed up for a short time, such as 15 seconds, when thesystem is first turned on by powering them steadily at moderate powerbefore commencing time-modulated operation. If the light sources 302,304 are designed to be run AC, successive high pulses should be ofopposite polarity. During the high phase the instantaneous current islarger than nominal, and must be carefully regulated. It is also knownin the art that current can be controlled by driving a light source withvery short bursts of pulses, each pulse in a burst lasting for about 0.1milliseconds, with a significantly longer interval between bursts.Alternatively, rather than driving the light sources 302, 304 of thepresent invention at a uniform 2× voltage during its powered-on phase,one can instead drive it with a succession of 4× pulses spaced apart by0.1 msec, and of 0.1 msec duration. The shape of the driving waveformcan be adjusted as necessary to facilitate re-ignition, and maintainelectrode lifetime. For example, the arc position of the light sources302, 304 can be stabilized by modulating the driving waveform (left onat low amplitude between bursts) with a period of about 0.03 msec.

Controller 312 runs light sources 302, 304 out of phase with oneanother, so that light source 304 is off when light source 302 is on, asshown in FIG. 3a, and light source 302 is off when light source 304 ison, as shown in FIG. 3c. Controller 312 also runs beam combiner 314 insynchronism with light sources 302, 304. Beam combiner 314 is preferablya digital mirror device (DMD), consisting of an array of mirrors 316which are switched back and forth between first and second tiltedpositions. Such devices are available commercially from TexasInstruments Inc. (TI) and typically include electronics for individuallyaddressing mirrors 316, and for driving them in a time sliced fashion sothat they spend an adjustable fraction of each {fraction (1/60)}thsecond video frame in one of the first or second tilt positions comparedto the other tilt position. To accommodate this time slicing, themirrors 316 in the DMD 314 can be switched in microseconds. TI DMDs 314are used as light valves with high pixel count, so the mirrors 316 canbe quite small (17 microns). Small size is important for thoseapplications where the DMD 314 must be switched in microseconds.However, none of this functionality is necessary for the presentinvention. Controller 312 simply switches all the mirrors 316 togetherin synchronism with the light sources 302, 304 output. When light source302 is on, as shown in FIG. 3a, the mirrors 316 are tilted at angle θtowards light source 302, such as 10 degrees (by way of example only).Light source 302 is preferably tilted at twice this angle, e.g. 20degrees, so that the mirrors 316 reflect the light source output intobeam 306 that is perpendicular to the substrate of combiner 314. Thetilt angle θ of the mirrors 316 is chosen large enough that all raysfrom light source 302 are incident on combiner 314 from the same side ofthe normal direction 318 a (318 b in FIG. 3c) to the mirrors 316. Thetilt angle α of the light sources 302, 304 is preferably chosen to betwice the mirror angle θ. Because light source 302 is driven at 2×instantaneous power during this portion of the cycle, the instantaneousintensity of beam 306 is double that which would be obtained if lightsource 302 were driven at a constant level equal to the maximumallowable time averaged power. This is illustrated schematically bycurve 320 a in FIG. 3b where the length of dotted line 322 a representsthe diameter of the lens used (not shown).

During the next video frame, light source 304 is switched on, lightsource 302 is switched off, and mirrors 316 are tilted in the oppositedirection, as shown in FIG. 3b. A simple way to improve efficiency is toensure that the mirrors 316 can switch in a time short compared to theduration of the video frame, though if such fast switching is notachieved, compensation can be made by adjusting the light sourcemodulation (see below). For millisecond switching speeds, the mirrors316 can be coarser than the 17 microns in TI's DMD 314. In the switchedposition shown in FIG. 3b, mirrors 316 now direct the output of lightsource 304 into the common output beam 306 direction. Output beam 306continues to have twice the intensity that these light sources 302, 304could ordinarily provide as shown by lines 320 a, and 320 b in theintensity graphs illustrated in FIGS. 3b and 3 d, respectively. However,the width and angular divergence of the output beam 306 are onlyslightly larger than that from a single source. Light sources 302, 304are each preferably driven at the maximum allowable power, as measuredover intervals comparable to the thermal time constant of theirelectrode structure. Light source power exceeds the steady-stateoperating limit only on time scales shorter than about 100 msec, andpreferably not longer then the video frame time, such as about 15 msec.

If light sources 302, 304 do not have precisely equal voltage responses,the signal initially received by detector 324 will be time varying. Thissignal can be used to balance the light sources 302, 304 drivingvoltages. Alternatively, light sources 302, 304 can be driven at asufficiently high rate, for example at double the video frame rate, thatany modulation in the output beam 306 is imperceptible.

Referring now to FIGS. 4a-4 d, there is illustrated a second embodimentof the illumination apparatus of the present invention, generallyreferred to by reference numeral 400. Modulator 314 in the firstembodiment shown in FIGS. 3a-3 d may only be available “off-the-shelf”if purchased as a DMD lightvalve. In this case it will likely containsophisticated functionality not needed for the present application, asnoted above. FIGS. 4a-4 d show an embodiment of the present inventionfor applications requiring polarized light in which components optimizedfor the present application are readily obtainable. During the firstvideo frame, and succeeding odd frames, shown in FIG. 4a, controller 402drives first light source 404 at 2× instantaneous power via supply 406.During odd cycles second light source 408 is either off or operated atminimum intensity. The peak brightness region of the P polarizedcomponent of the first light source 404 is transmitted by polarizingcoating 410 as beam 412. The peak brightness S component is reflected bycoating 410 and folded by mirror 414 to form beam 416. Static retarder418 rotates the polarization of beam 416 to the same P state as beam412. The asymmetric alignment of the first light source 404 and mirror414 relative to beamsplitter 420 allows lens 422 to collect the highbrightness “hot spots” of beams 416 and 412. However, the instantaneousintensity of the beam collected by lens 422 is twice as high as thatcollected by lens 120 in FIGS. 1a and 1 c, because the first lightsource 404 is operated at 2× higher power during its “on-phase” in the50% duty cycle.

During the other half of the duty cycle, corresponding to the even videoframes and shown in FIG. 4c, the first light source 404 is substantiallyoff while the second light source 408 is operated at 2× intensity bycontroller 402 via power supply 407. Coating 410 now causes beam 424 tobe S polarized, since the beam is now incident from the second lightsource 408 in reflection. Beam 426 is likewise S polarized after exitingretarder 418. However, during the even frames controller 402 switches onmodulator 428, so that the output beams 424, 426 are rotated to P state.During the odd frames, when beams 412 and 416 are P polarized asdescribed above, modulator 428 is switched off, leaving the output beams412, 416 in P state for the odd frames as well.

Thus, a continuous P polarized beam of 2× intensity is produced.Alternatively, a continuous S output can be produced by reversing thephase of modulator 428, or by moving static rotator 418 from beam 416 tobeam 412.

As with the previous embodiment, if light sources 404, 408 do not haveprecisely equal voltage responses, the signal initially received bydetector 434 will be time varying. This signal can be used to balancethe light sources 404, 408 driving voltages. Alternatively, lightsources 404, 408 can be driven at a sufficiently high rate, for exampleat double the video frame rate, that any modulation in the output beams412, 418 or 424, 426 is imperceptible.

The terms “on” and “off” in reference to modulator 428 merelydistinguish the action of the modulator in rotating the outputpolarization. In the “off” state modulator 428 leaves the polarizationnominally unrotated; in this state the polarization might actually berotated 180° without affecting the operation of the invention. In the“on” state modulator 428 rotates the polarization 90°. Modulators torotate the polarization 180° or 0° (“off”), or 90° (“on”) can be basedon liquid crystal (LC) effects, such as the tunable birefringenceeffect, the wave-guide effect, and the surface-stabilized ferroelectricliquid crystal (FLC) effect, each of which is next described.

The Tunable Birefringence Effect

Two forms of the tunable birefringence effect can be used, thehomogeneous (or parallel aligned) LC cell and the Π-cell. In eithercase, the modulator 428 is a cell containing nematic LC medium betweentwo electrodes for switching, and in both cases the LC directorsadjacent to the cell substrates have a small pretilt angle from thesubstrate plane. In Π-cells the LC directors are aligned to have areflection symmetry with respect to the central plane of the LC cell.The LC directors in the homogeneous LC cell are parallel to each otherin the quiescent state. In both cells the nematic LC mixtures havepositive dielectric an isotropy, so that by changing the applied voltageacross the cells the retardance can be tuned from an initial value of2ΠdΔn/λ to almost zero, where d, Δn, and λ are the cell gap, effectivebirefringence, and wavelength of the incident light, respectively. Aslong as 2ΠdΔn/λ>1 there will always exist two switchable states in whichthe input polarization is rotated by 90° in one and 180° in the other.

The Wave-Guide Effect

A 90° twisted nematic (TN) LC cell can be used to build the modulator428 based on the wave-guide effect. In a 90° TN cell the directors twist90° from one cell substrate to the other. The input polarization will berotated 90° in the voltage-off state if the input polarization isparallel (or perpendicular) to the LC directors near the entrancesurface, and if dΔn/λ is set equal to n^(2·)−0.25, where n is aninteger. The other (almost) unrotated state is achieved by applying alarge enough voltage to the cell that most of the LC directors arealigned parallel to the electric field, eliminating the wave-guideeffect.

The Surface-Stabilized FLC Effect

A surface-stabilized ferroelectric LC cell can also be used to rotatethe input polarization by 90° or 0°. The directors in the FLC cell havetwo stable positions, switchable by applied electric field. One of thestable positions can be set parallel to the input polarization, sopolarization is not rotated when the cell is switched to this state. IfdΔn/λ is set to 0.5, the FLC cell becomes a halfwave retarder. Whenswitched to the second stable state the retarder axes rotate through anangle 2β, where β is the so-called half-cone-angle of the FLC molecule.If β=22.5°, the polarization is rotated 90° when the cell is switched tothe second state.

These modulators 428 provide switching times in approximately themillisecond regime. This is fairly short compared to the 17 msec frameduration of a display operated at 60 Hz. However, if the modulator 428is in transition during part of the time that a light source is pulsedhigh, some portion of the output beams 412, 416 or 424, 426 will brieflybe switched by the modulator 428 to the wrong polarization. Projectionsystems usually have a supplementary polarizer to trim small backgroundcomponents in the wrong polarization, but the trimmed light nonethelessrepresents wasted power. To improve efficiency, each source can bepulsed high with a duty cycle slightly lower that 50%. For example, ifthe modulator has a 1 msec switching time, then in two successive cycleslasting a total of 34 msec, one of the first or second light sources404, 408 might be pulsed high between 0.5 and 16.5 msec, and the otherlight source between 17.5 and 33.5 msec. In order to run the lightsources 404, 408 at nominal rated power in the time-average, theinstantaneous power should be increased by 2.12×, instead of the 2×increase that applies with a simple 50% duty cycle. While it isimportant that the time-averaged power not exceed the nominal rating,with some light sources 404, 408 it is also desirable that thetime-averaged power not be substantially lower than the nominal rating;i.e. the power level in some light sources must be held fairly close tonominal.

When the above conditions are met, the embodiment of FIGS. 4a-4 dprovides an approximately 2× brighter beam than the prior artarrangement of FIGS. 1a-1 d. However, the beam it provides is not fullyoptimized in its brightness distribution. Mirror 414 and retarder 418 doprovide some improvement in the brightness distribution beyond that ofthe raw beam emitted from the light sources 404, 408, because in theembodiment of FIGS. 4a-4 d, lens 422 is able to collect from the rawbeam two zones of high brightness, as illustrated in graphs 430 a and430 b in FIGS. 4b and 4 d, respectively. However, further improvement ispossible; it is possible to increase the inhomogeneity of the beam,making the brightness of the central hot spots more pronounced, byfocusing the light onto the region of coating 410, mirror 414, and lens422, i.e. focusing the light onto the region where the beam istruncated. Of course, the simple “zero-order” light concentrationprovided by focusing does not improve brightness. Since brightnessrefers to projected optical power per unit solid angle per unitprojected source area, the basic action of focusing, namely to produce abeam that is more concentrated but also proportionately more divergent(or convergent) does not in itself increase brightness. However, withmany light sources the detailed shape of the focused beam image willhave a more pronounced peak than does the distribution immediately infront of the light source.

Referring now to FIG. 5, there is illustrated a third embodiment of theillumination apparatus of the present invention, generally referred toby reference numeral 500, in which the same or similar elements arereferred to like reference numerals from the previous embodiment. FIG. 5shows an arrangement in which the brightness of the two hot spotscollected by lens 502 is slightly increased over the embodiment shown inFIGS. 4a-4 d, because the reflectors of the light sources 404, 408 aremade to focus the beams in the vicinity of mirror 414 and lens 422 wherethe spatial limits of the collected beam are defined. The light sourcesbeams 502, 504 should preferably not be strongly focussed, but insteadshould be converged with an angle not larger than about +/−10 degrees,since polarizing coating 410 would not typically work well over a largerrange. Mirror 414 and lens 422 can be physically smaller than those usedin the embodiment of FIGS. 4a-4 d,since, for given optical systemetendue, the widened angular range of the beam means that the portioncollected by the optical system must have a smaller diameter.

Because of the +/−10 degree beam angle limit, the light sources 404, 408in the FIG. 5 embodiment must be drawn back fairly far from beamsplitter420. In some cases this can make for an unwieldy optical system. Thereare known optical arrangements in the art for focusing light sourcesonto beamsplitters in a compact way, and this is also possible withtime-modulated light sources.

Referring now to FIG. 6, there is illustrated a prior art optical systemin which an incident light beam (shown schematically as arrow 622) isfocussed by an array of lenses 623 onto a second array of lenses 624,onto a beamsplitter array 625, then onto a condenser lens 626, andfinally projected onto an illumination field 627. Because the individualfocussing lenses 623 a, 624 a are small in diameter, they can focuslight into beamsplitter array 625 at the desired small angles withoutbeing separated from it by a large distance.

Referring now to FIGS. 7a and 7 b, there is illustrated a side view oflens array 623 and second lens array 624. For clarity, beamsplitterarray 625 is not shown. Beam 622 is nominally collimated, but because ofbrightness limitations in the source it will inevitably contain lightpropagating in a range of angles, for example a +/−2 degree range. FIG.7a shows how each lenslet 623 a in array 623 focusses an image of thesource in the vicinity of the corresponding lenslet 624 a of array 624,e.g. from lenslet 623 a an image is focussed onto lenslet 624 a. FIG. 7bshows by dashed lines the behavior of a bundle of rays 629 in nominallycollimated beam 622 that is incident on lenslet 623 a at an angle, suchas 2 degrees. Rays 629 focus to an off-center point in the arc image onlenslet 624 a, whereas rays 628 (shown solid) focus at the center oflenslet 624 a. Though dashed rays 629 are initially tilted relative torays 628, the second lenslet 624 a refracts bundle 629 into a directionparallel to bundle 628. For this to be accomplished lenslet 624 a mustbe given a focal length substantially equal to the separation betweenarrays 623 and 624, which is also the approximate focal length of array623 (if beam 622 is collimated). At the exit (left) side of array 624all such bundles from matched lenslet 624 a are thus rendered parallel.If the light source is reasonably well matched to the collectionaperture of the optical system, the illumination will be in thetransitional range between the purely power-limited and purelybrightness-limited regimes, and the high brightness region of the arcimage at lenslet 624 a will fill a reasonably large fraction of thelenslet area, such as half or more.

Thus, light of appreciable intensity will be emitted by many points onthe output face of array 624, and the light emitted by all such pointswill consist of cones that are rendered parallel by the lenslets inarray 624. These cones will not have a circular cross-section, but willinstead have a cross-section corresponding to that of the lenslets 623 ain array 623, such as rectangular or square, as shown in perspective inFIG. 6.

Referring to FIGS. 6 and 7c, illumination field 627 is located at thefront focal plane of condenser lens 626, while lenslet array 624 istypically located at approximately the back focal plane. Since the conesof light emitted by condenser 624 are parallel and of common rectangularcross section, the output surface of array 624 can be considered to emita collection of ray bundles, with each ray bundle composed of parallelrays, and where the parallel rays in a bundle correspond to a particulardirection within the common rectangular cross-section of the cones. Forexample, the rays in cones like 628 and 629 in FIG. 7c can also bedivided into parallel bundles, such as the bundle of parallel raysincluding the pair labeled 633, or the parallel bundle including thepair labeled 634. Each bundle contains parallel rays emitted fromsubstantially every point on array 624.

Since illumination field 627 is at the front focus of condenser 626, anybundle of parallel rays input to lens 626 will be focused to a singlepoint on illumination field 627. Thus, bundles 633 and 634 in FIG. 7ceach focus to different points on field 627. Since an input bundle ofparallel rays is present for every direction within a cone ofrectangular cross section, the collection of focussed illuminationpoints at 627 which are formed from the input bundles will have amatching rectangular cross section. The illuminated field at 627 willthus have the same aspect ratio as the lenslets 623 a in array 623. Inother words, condenser lens 626 converges the parallel cones of lightthat diverge from array 624 to a common rectangular overlap atillumination field 627.

If array 624 is approximately at the rear focus of condenser lens 626,the rays within a single cone will be approximately parallel when theyare incident from a particular direction on rectangular illuminationfield 627. The array of source images formed on array 624 will thenrepresent a map of the common set of directions from which light is madeto illuminate field 627. Each point on illumination field 627 within theilluminated rectangle receives light from this common set of directions.When every point on field 627 is illuminated from the same set ofdirections the illumination is termed telecentric; this is often (butnot always) desirable. Illumination field 627 should be placed at thefront focus of condenser lens 627 to satisfy the basic requirement thatthe cones be overlapped; if telecentric illumination is also desired,array 624 should be placed approximately at the rear focal plane ofcondenser lens 626.

If the illumination on field 627 is relayed into a projection system,the range of illuminating directions and the size of rectangular field627 can be matched to the etendue of the projection optics. Ifilluminating beam 622 in FIG. 7b is derived from a light sourceoperating in the transitional region between the purely power-limitedand purely brightness-limited regimes, then a modest intensity will bepresent in beams like 636 in FIG. 7b (shown dot-dashed) that enterlenslets like 623 a at such steep angles that they are not collected bythe matched lenslet, e.g. by lenslet 624 a. These cones emerge from amismatched lenslet 624 b of array 624 at such steep angles that they arelost. They therefore represent source light that cannot be collectedwithin the optical system etendue. However, rays from the highestbrightness regions of the focussed source images at array 624, such ascones 628 and 629, are collected.

FIG. 7d illustrates the effect of placing beamsplitter array 625 next toarray 624. When beams such as 628 enter beamsplitter 625 they strike oneof a set of parallel interfaces coated with polarizing filters, such ascoating 605 a. The P polarized component is transmitted through coating605 a, but is transformed to S polarized light by halfwave retarder 609.The S polarized component of beam 628 is reflected downward by coating605a, but is reflected again into the forward direction by the adjacentcoating 605 b. Retarder 609 and adjacent retarders are stripe-like inshape, and the S component bypasses them through the separating space toemerge as S polarized beam 634. Cone 634 has the same rectangular crosssection as the directly transmitted cone 628 because coatings 605 a and605 b are parallel. Because they share parallel rectangularcross-sections, both the S and the P-converted-to-S components of theinput light are focussed to field 627 (as S polarized light). Cone 634emerges from a slightly different focal plane, giving rise tomicro-distortions in the range of directions illuminating field 627, butthese are usually of no consequence.

Dimmer cones that emerge from lenslets 624 a away from the brightcentral region, such as cone 637 (shown dotted in FIG. 7d), are blockedby opaque screen 635. If left unblocked, the P component of such lightwould pass through the space between retarders such as 609 and so wouldexit as P polarization, no longer matching the S polarization of e.g.beam 628. However, cone 637 is relatively dim, and beamsplitter array625 essentially substitutes for it the high brightness S component 634of beam 628. The output therefore consists primarily of rays from thehigh brightness portion of the input, and these high brightness rays areemphasized by the focussing of the light near array 625. Since thebrightness of the beam is almost maximized with a well-matched source,it is not straightforward to increase output power beyond that achievedin the FIG. 6 system.

Referring now to FIGS. 8a-8 d, there is illustrated a fourth embodimentof the illumination apparatus of the present invention, generallyreferred to by reference numeral 800 wherein two time-modulated lightsources 801, 802 are utilized to increase beam output power withoutincreasing beam diameter or angular divergence. the operation ofillumination apparatus 800 will be clearer from reference to FIGS. 8band 8 d where subsets of the components are shown, primarily thoserelating to one of the light sources 801 in FIG. 8b, 802 in FIG. 8d. InFIG. 8b light source 801 illuminates lenslet array 823 a with a beamthat is nominally collimated, but which must nonetheless exhibitnon-negligible divergence when a light source achieving useful outputpower is chosen. The brightest region of the arc images projected bylenslets 830 a of array 823 a will be received in the central regions ofthe corresponding lenslets 831 a of second array 824 a. The lenslets 831a in array 824 a are preferably molded into the front surfaces of lightguides 538, as shown in FIG. 8b. Lenslets 831 a may alternatively becombined into a single element (similar to element 424 shown in FIG. 6).Light guides 838 are solid slabs of optical material, such as glass,contacted at one face to PBS array 825. The thickness of each slabapproximately equals the thickness of PBS array 825; retarder strips 809are also spaced apart by approximately the same distance.

Referring now to FIG. 8c, the light guides 838 and PBS array 825 ofillumination apparatus 800 are shown in greater detail. As discussedpreviously with reference to lenslet 624 a of FIG. 7b, light emergesfrom lenslet 831 a as a set of parallel cones. Because these cones areparallel, the total light beam entering light guide 838 will fill arange of directions that is rectangular in cross-section. The aspectratio of this cone of directions will match the cross-section oflenslets 830 a in array 823 a of FIGS. 8a and 8 b. As discussed furtherbelow, this rectangular range of directions would typically subtend oforder ±10°. Light guide 838 has plane parallel side faces 838 a, 838 b,as shown in FIG. 8c. These faces are parallel to the central axis of therectangular cone of directions that is input to the guide 838, and havethe same horizontal and vertical orientations as the edges of lenslet830 a. This means that rays which reflect off the side faces 838 a, 838b will continue to be contained within the same rectangular range ofdirections after reflection, because the image of the rectangularcross-section as mirrored in a side face is still the same rectangle.Moreover, as long as the range of directions is less than ˜45°, theserays will undergo total reflection at the side faces 838 a, 838 b, andno light will be lost out the sides 838 a, 838 b of the guide 838. Thesame considerations apply for reflections from the two narrow side facesnot visible in the FIG. 8c view.

Light therefore exits guide 838 into PBS 825 with the same rectangularrange of directions as it has when it enters guide 838 from lenslet 831a. Rays within the rectangular range that enter guide 838 through aparticular point of lenslet 831 a, such as cone 839 (shown dotted inFIG. 8c), will continue to subtend this range of directions when theyemerge from guide 838, even though, as FIG. 8c shows, they will notgenerally exit guide 838 through a single common point. Similarly, therays emitted from a single point at the exit of the guide, such as rays828, will fill this rectangular cone, even though in general these rayswill have entered the guide through different points, i.e. as parts ofdifferent cones from lenslet 831 a. This is illustrated for rays 828 inFIG. 8c.

Because guides 838 emit parallel cones of light such as 828 that have acommon rectangular cross-section, PBS array 825 and lenslet array 824 acan carry out much the same function as do adjacent lenslet array 624and PBS array 625 illustrated in FIGS. 6 and 7, despite the fact that inthe FIG. 8c embodiment, the PBS array 825 and lenslet array 824 a areseparated by guides 838. The P polarized component of beam 828 istransmitted through polarizing coating 805 a. It then passes through thespace between adjacent retarder elements 809 to emerge as a P polarizedbeam. The S polarized component of cone 828 is reflected from coatedsurface 805 a as beam 834 (shown dot-dashed in FIG. 8c); it thenreflects from the neighboring coating 805 b to emerge from PBS array 825through retarding element 809. Element 809 has substantially halfwaveretardance, and so rotates the polarization of beam 834 from S to P. Thelight from guides 838 therefore emerge from PBS array 825 as a set ofparallel cones in P polarization. As illustrated in FIG. 7d, dimportions of the light collected by the lenslets 623 a of array 623 areblocked by shields 635 in order to make room for polarization-convertedhigh brightness beams like 634 to be inserted into the output light. Inthe illumination apparatus 800 of FIG. 8c, these dim input portions arenot collected within the thickness of guides 838, but are lost in thespaces 840 between. Similar shields (not shown) can be inserted in thespaces 840 between lenslets 831 a to prevent stray light.

Because the rectangular cones emerging from PBS array 825 are parallel,they will overlap at the focus of condenser lens 826 in FIG. 8b, therebyilluminating rectangular field 827 with a uniform patch of P polarizedlight.

To accomplish this illumination the dimensions and focal lengths of theFIG. 8b components are preferably chosen by methods that are well knownto those skilled in the art. If the beam from light source 801 havingwidth D_(lamp) is divided along one axis into m segments by the lensletsin array 824 a, we have:

D_(fly's-eye)=D_(lamp) /m

where D_(fly's-eye) is the width of a lenslet element. It then followsfrom the optical invariant that${{NA}_{{fly}^{\prime}s\text{-}{eye}} = {m\frac{D_{object}\quad {NA}_{proj}}{D_{lamp}}}},$

where D_(object) is the width of the object at illuminated field 827 andNA_(proj) is the sine of the angular range illuminating this object. Theseparation between arrays 823 a and 824 a is approximatelyD_(lamp)/2mNA_(fly's-eye), and the focal length of condenser lens 826 isapproximately D_(object)/2NA_(fly's-eye). Thus, the projector becomesmore compact and the illumination more uniform when m (and thereforeNA_(fly's-eye)) are increased. However, the focal length of condenserlens 826 must be kept large enough to accommodate other optics that maybe needed between the object and illuminator, such as color separatingprisms(not shown). Furthermore, NA_(fly's-eye) should be kept below ˜10°for efficient performance from the coatings in PBS array 825.

Referring now to FIG. 8d, there is illustrated a different subset of theFIG. 8a embodiment. Light guides 840 are slabs of optical material,preferably glass, like guides 838 in the FIG. 8b subset, but guides 840are cut in the shape of 45° right triangles. One side of the right angleis positioned against PBS 825 with lenslet array 823 b being adjacent tothe other side. The lenslet elements 831 b of array 823 b are preferablymolded into guides 840, as shown in FIG. 8d, or alternatively can becontacted together to form a single freestanding element. The thicknessof guides 840 is preferably approximately equal to the thickness of PBSarray 825 and to the spacing between retarder strips 809. FIG. 8a showsthe full illumination apparatus 800, in which the triangular guides 840of FIG. 8d are interleaved with the rectangular guides 838 of FIG. 8b.Only a very small air space is needed between the guides in order toconfine the light, but the gap could be made as large as ˜0.1 mm withoutappreciably impacting light collection into the guides.

The triangular guides shown in FIG. 8d serve to reflect the light fromlight source 802 into PBS array 825. The 45° hypotenuse faces thataccomplish this reflection would not usually need a mirror overcoatingfor high reflectivity; no light will be transmitted through these facesso long as NA_(fly's-eye)<({square root over (n²−1+L )}1−1)/{square rootover (2)}. As discussed further below, NA_(fly's-eye) would typically be˜0.1, so any glass of reasonably high index would provide totalreflection. Guides 840 should also be separated by a very small air gapfrom PBS array 825. If instead the guides were directly coupled to thePBS, a few rays from the closest lenslets in array 823 b to PBS 825would enter the array obliquely, without being reflected from thehypotenuse.

Guides 840 are aligned with retarder strips 809, while guides 838 arealigned with the spaces separating these retarder strips 809, as shownin FIG. 8a. This means that the directly transmitted P polarizedcomponent of the light from guides 840 will be converted to Spolarization by the retarders, while the component that is initially Swill exit PBS array 825 through the spaces separating the retarders, andwill therefore remain in S polarization. All collected light from lightsource 802 therefore emerges from PBS array 825 as S polarization.

In contrast, light from light source 801 emerges from PBS array 838 as Ppolarization. Illumination apparatus 800 is similar in this respect tothe illumination apparatus 400 illustrated in FIGS. 4a-4 d, and followsa similar procedure to superimpose the beams in a common polarization(e.g. S). During odd video frames, light source 801 is cycled toapproximately 2× instantaneous power, while during even frames it isturned off, or substantially off. Conversely, light source 802 issubstantially off during odd frames, and is cycled to 2× instantaneouspower during even frames. Both light sources 801, 802 are therefore runat rated power in the time average. During odd frames, modulator 821rotates the 2× P polarized output from light source 801 to Spolarization. Rotation from modulator 821 is switched off during evenframes to leave the 2× S polarized output of light source 802 in Spolarization. An S polarized beam of 2× intensity therefore enterscondenser lens 826 during both the even and odd frames. Illuminationapparatus 800 captures approximately the same intensity from two lightsources that the FIG. 6 system of the prior art captures from one, yetit delivers this 2× intensity without increasing the width or divergenceangle of the beam. As in the FIG. 6 system, one would naturally choosefor illumination apparatus 800 light sources with power levels in theoptimum range. Light sources with a significantly larger than optimumoutput power would not be able to deliver the extra power to aprojection system with the given lightvalve size and projection lens NA.Thus, the 2× increased power illumination apparatus 800 cannot beachieved in the FIG. 6 system of the prior art by simply employing amore powerful light source.

The descriptions of the various embodiments given thus far have involvedthe combination of beams from two light sources into a common outputbeam (though the embodiment of FIG. 3 is not restricted to two lightsources). However, these embodiments can all be staged in order tocombine more than two light sources. For example, two subsystemsaccording to the above embodiments can each combine two light sourcesinto an output beam, and the two output beams from the two subsystemscan then be combined by a third subsystem to produce an output beamcommon to all four light sources. Each light source would be cycledsequentially to 4× instantaneous intensity with a 25% duty cycle. Thetwo output beams from the first two subsystems would then have 4×instantaneous intensity with a 50% duty cycle, and the overall outputbeam would be emitted continuously at 4× intensity.

While there has been shown and described what is considered to bepreferred embodiments of the invention, it will, of course, beunderstood that various modifications and changes in form or detailcould readily be made without departing from the spirit of theinvention. It is therefore intended that the invention be not limited tothe exact forms described and illustrated, but should be constructed tocover all modifications that may fall within the scope of the appendedclaims.

Having thus described our invention, what we claim as new, and desire tosecure by Letters Patent is:
 1. An illumination apparatus comprising: atleast two light sources, each of which produce independent light beams,a controller for sequentially driving each of the light sources at ahigh power above their respective maximum rated power, to produce arespective light beam for each light source, and for leaving theremaining light sources at a low power below their respective maximumrated power, such that the time-average of the high and low power levelsare set to a predetermined value for each of the light sources, andcombining and directing means for sequentially combining each of thelight beams from their respective light sources while being driven athigh power into a common output beam with a fixed direction.
 2. Theillumination apparatus according to claim 1, wherein the combining anddirecting means comprises an array of tiltable mirrors that sequentiallydirect each of the light beams from their respective light sources whilebeing driven at high power into a common output direction.
 3. Theillumination apparatus according to claim 2, wherein the at least twolight sources comprise first and second light sources, and wherein thearray of tiltable mirrors are capable of achieving first and secondtilted positions relative to the array of tiltable mirrors andcorresponding to the first and second light sources, respectively, thearray of tiltable mirrors being tilted towards their respective lightsources while being driven at the high power.
 4. The illuminationapparatus of claim 3, wherein the first and second light sources arefurther tilted towards the array of tiltable mirrors.
 5. Theillumination apparatus of claim 4, wherein the first and second lightsources are tilted at an angle twice that of its respective tiltedposition.
 6. The illumination apparatus according to claim 1, furthercomprising a switchable polarization rotator that sequentially convertsthe polarization of the light beams produced by each of the lightsources into a common polarization.
 7. The illumination apparatusaccording to claim 6, wherein the operation of the switchablepolarization rotator is based on a tunable birefringence effect.
 8. Theillumination apparatus according to claim 6, wherein the operation ofthe switchable polarization rotator is based on a wave-guide effect. 9.The illumination apparatus according to claim 6, wherein the operationof the switchable polarization rotator is based on a surface-stabilizedferroelectric liquid crystal cell effect.
 10. The illumination apparatusaccording to claim 1, wherein the predetermined value for each of thelight sources is their respective maximum rated power.
 11. Theillumination apparatus according to claim 10, wherein two light sourcesare used, and wherein the high power is two times the maximum ratedpower of the light sources and the low power is substantially zero. 12.The illumination apparatus according to claim 1, wherein the lightsources are metal halide lamps.
 13. The illumination apparatus accordingto claim 1, wherein the controller also controls the combining anddirecting means for coordinating the sequential driving of the lightsources and the sequential combining and directing of the light beamsfrom their respective light sources.
 14. The illumination apparatusaccording to claim 1, further comprising a detector for detecting theintensity of the light beams and inputting the controller to modify thepower to the light sources in response thereto.
 15. The illuminationapparatus according to claim 1, wherein the at least two light sourcescomprise first and second light sources in which the produced lightbeams from each light source having a first and a second polarization,and wherein the combining and directing means comprises an opticalsystem.
 16. The illumination apparatus according to claim 15, whereinthe optical system comprises: a polarizing beamsplitter for transmittinglight of the first polarization and reflecting light of the secondpolarization, a fold mirror collecting either the transmitted orreflected light from the beamsplitter and directing the collected lightin a direction parallel to the other light, a half-wave retarderrotating either the transmitted or reflected light from thebeamsplitter, and a condenser lens collecting the parallel transmittedand reflected light from the beamsplitter.
 17. The illuminationapparatus according to claim 16, further comprising a detector fordetecting the intensity of the light beams and inputting the controllerto modify the power to the light sources in response thereto.
 18. Theillumination apparatus according to claim 16, further comprising aswitchable polarization rotator for sequentially converting thepolarization of the transmitted and reflected light produced by each ofthe first and second light sources into a common polarization.
 19. Theillumination apparatus according to claim 18, wherein the operation ofthe switchable polarization rotator is based on a tunable birefringenceeffect.
 20. The illumination apparatus according to claim 18, whereinthe operation of the switchable polarization rotator is based on awave-guide effect.
 21. The illumination apparatus according to claim 18,wherein the operation of the switchable polarization rotator is based ona surface-stabilized ferroelectric liquid crystal cell effect.
 22. Theillumination apparatus according to claim 18, wherein the controlleralso controls the switchable polarization rotator for coordinating thesequential driving of the light sources and the sequential conversion ofthe polarization of the transmitted and reflected light produced by eachof the first and second light sources into a common polarization. 23.The illumination apparatus according to claim 16, wherein the lightsources are configured such that the light beam from each respectivelight source is made to substantially focus in the vicinity of the foldmirror and condenser lens.
 24. The illumination apparatus according toclaim 15, wherein the optical system comprises: first and second sets oflight guides corresponding to the first and second light sources, eachindividual light guide having two pairs of substantially parallelsurfaces, first and second pairs of lenslet arrays corresponding to thefirst and second light sources for focusing the light from individuallensets into a corresponding individual light guide, a polarizing beamsplitter array for directly transmitting the light of the firstpolarization from each of the first and second sets of light guides, andtransmitting the light of the second polarization from the first andsecond sets of light guides after reflection from the substantiallyparallel surfaces, and half-wave retarding strips for converting eitherthe directly transmitted light of the first polarization or thetwice-reflected light of the second polarization to match the other ofthe polarization components.
 25. The illumination apparatus according toclaim 24, further comprising a switchable polarization rotator forsequentially converting the polarization of the beams produced by eachof the first and second light sources into a common output polarization.26. The illumination apparatus according to claim 25, wherein theoperation of the switchable polarization rotator is based on a tunablebirefringence effect.
 27. The illumination apparatus according to claim25, wherein the operation of the switchable polarization rotator isbased on a wave-guide effect.
 28. The illumination apparatus accordingto claim 25, wherein the operation of the switchable polarizationrotator is based on a surface-stabilized ferroelectric liquid crystalcell effect.
 29. The illumination apparatus according to claim 25,wherein the controller also controls the switchable polarization rotatorfor coordinating the sequential driving of the light sources and thesequential conversion of the polarization of the transmitted andreflected light produced by each of the first and second light sourcesinto a common polarization.
 30. The illumination apparatus according toclaim 24, further comprising a detector for detecting the intensity ofthe light beams and inputting the controller to modify the power to thelight sources in response thereto.
 31. The illumination apparatusaccording to claim 24, wherein the first and second light guides areinterleaved with each other to produce a compact illumination system.32. A method for increasing illumination in an illumination apparatus,the method comprising the steps of: producing independent light beams,from at least two light sources, sequentially driving each of the lightsources at a high power above their respective maximum rated power forproducing the respective light beam for each light source, while leavingthe remaining light sources at a low power below their respectivemaximum rated power, such that the time-average of the high and lowpower levels are set to a predetermined value for each of the lightsources, and sequentially combining each of the light beams from theirrespective light sources while being driven at high power into a commonoutput beam with a fixed direction.
 33. The method according to claim32, wherein the combining step comprises titling an array of tiltablemirrors for sequentially directing each of the light beams from theirrespective light sources while being driven at high power into a commonoutput direction.
 34. The method according to claim 32, furthercomprising the step of sequentially converting the polarization of thelight beams produced by each of the light sources into a commonpolarization.
 35. The method according to claim 32, further comprisingthe steps of detecting the intensity of the light beams and inputtingthe controller to modify the power to the light sources in responsethereto.
 36. The method according to claim 32, wherein the illuminationapparatus is used in a projection display and wherein the method furthercomprising the steps of: displaying sequential image frames on at leastone lightvalve, illuminating the at least one lightvalve with the commonoutput beam by which the displayed sequential image frame is reflected,and projecting the displayed sequential image frames of the at least oneilluminated lightvalve onto a screen.
 37. An illumination apparatuscomprising: at least two light sources, each of which produceindependent light beams, a controller for sequentially driving each ofthe light sources at a high power above their respective maximum ratedpower, to produce a respective light beam for each light source, and forleaving the remaining light sources at a low power below theirrespective maximum rated power, such that the time-average of the highand low power levels are set to a predetermined value for each of thelight sources, combining and directing means for sequentially combiningeach of the light beams from their respective light sources while beingdriven at high power into a common output beam with a fixed direction,said combining and directing means comprising an array of tiltablemirrors that sequentially direct each of the light beams from theirrespective light sources while being driven at said high power,said atleast two light sources comprising first and second light sources, andwherein the array of tiltable mirrors are capable of achieving first andsecond tilted positions relative to the array of tiltable mirrors andcorresponding to the first and second light sources, respectively, thearray of tiltable mirrors being tilted towards their respective lightsources while being driven at the high power, and said first and secondlight sources being further tilted towards the array of tiltablemirrors.
 38. The illumination apparatus of claim 37, wherein the firstand second light sources are tilted at an angle twice that of itsrespective tilted position.
 39. The illumination apparatus according toclaim 37, wherein the predetermined value for each of the light sourcesis their respective maximum rated power.
 40. The illumination apparatusaccording to claim 39, wherein two light sources are used, and whereinthe high power is two times the maximum rated power of the light sourcesand the low power is substantially zero.
 41. The illumination apparatusaccording to claim 37, wherein the light sources are metal halide lamps.42. The illumination apparatus according to claim 37, wherein thecontroller also controls the combining and directing means forcoordinating the sequential driving of the light sources and thesequential combining and directing of the light beams from theirrespective light sources.
 43. The illumination apparatus according toclaim 37, further comprising a detector for detecting the intensity ofthe light beams and inputting the controller to modify the power to thelight sources in response thereto.
 44. A method for increasingillumination in an illumination apparatus, the method comprising thesteps of: producing independent light beams, from at least two lightsources, sequentially driving each of the light sources at a high powerabove their respective maximum rated power for producing the respectivelight beam for each light source, while leaving the remaining lightsources at a low power below their respective maximum rated power, suchthat the time-average of the high and low power levels are set to apredetermined value for each of the light sources, sequentiallycombining each of the light beams from their respective light sourceswhile being driven at high power into a common output beam with a fixeddirection, tiling an array of tiltable mirrors during said combiningstep for sequentially directing each of the light beams from theirrespective light sources while being driven at high power into a commonoutput direction, and further comprising the steps of detecting theintensity of the light beams and inputting the controller to modify thepower to the light sources in response thereto.